Discrimination and rapid identification of fleetingly small traces (down to single molecules) of chemicals from within fluctuating chemical backgrounds are the pervasive goals of analytical chemistry. A wide range of military, public, and private applications demand continued improvement in chemical detection methods: contraband (drugs and explosives) detection in the mail, in airports, at border crossings, in the schools and the workplace; forensics; chemical and biological defense (explosives, chemical and biological weapons); human and veterinary diagnostics; adsorption, deposition, metabolism, excretion, and toxicology studies conducted on human and veterinary therapeutics, agricultural chemicals, and in industrial biology; environmental fate; and bioinformatics and high throughput screening
Aerosolized chemical toxins, either from industrial or military release, pose a clear threat to military forces in many theaters of operation. Explosives (mines) and munitions detection is a critical military mission for chemical detectors. Military threats also include overt and covert use of conventional or new chemical warfare (CW) agents. Potential nonmilitary threats include: industrial pollution (e.g., in the Eastern Block and many developing nations) and collateral or intentional damage of industrial sites (e.g., the oil well fires set during Operation Desert Storm).
Current chemical detection systems depend upon the accumulation of a sufficient mass of agent in order to achieve detection above background, which limits their intrinsic sensitivity. Spectroscopic detection methods are often used to distinguish a known chemical species from fluctuating natural chemical backgrounds. Chemical specific probes, such as antibodies or molecularly imprinted adsorbents, have proved difficult to develop for small molecule organic compounds, leaving direct detection methods (e.g., surface acoustic wave devices, mass spectrometers, and optical systems) the only currently-viable methods to detect most chemical agents. This mass sensitivity issue also makes these detection systems difficult to miniaturize since sufficient mass can be difficult to accumulate in a small space, which means point sensors for chemical detection require conspicuous and expensive collection preconcentration systems.
Other major chemical detection applications for mass spectrometers include contraband and explosives detection; food, beverage, and cosmetic product quality control; food safety and quality assurance; and ventilation control (offices and airplanes). The Congressional Budget Office (CBO) estimated in 1997 that US governments at all levels spend $1 B/y on the care and training of sniffer dogs for the detection of contraband, explosives, or rescue operations in the public arena. (Congression Budget Office estimate reported in US News & World Report (November, 1997)). Prior to 2001 the FAA failed to adopt mass spectrometer based detection strategies at US airports because of their demonstrated lack of sensitivity (generally in the 1–100 fmole range for explosives).
One of the USPS' highest priority interests is in the detection of fraudulent or prohibited mailings. Ted Kazinski (the “Unibomber”) has once again highlighted the need for a broad, but sensitive screen, without intrusion. Mercury has been found in a parcel on-board an airplane. The catastrophic poisoning potential of such a material, following a leak during flight, could be devastating. In addition, biologic agents could also be addressed, which is a heightened issue with the recent outbreak of hoof-and-mouth disease in Europe. Among the materials with which the Postal Service concerns itself are marijuana, methamphetamine, cocaine and heroin.
Two key issues with which the USPS must concern itself, when reviewing and planning for systems integration of sensors and user-interfaces, include: false alarm rate (must be kept as low as possible) and impact on mail sorting and transporting throughput. MS detection systems would uniquely meet these requirements if it were not for their poor overall detection efficiency. The problem with MS-based sensors is the current need for comparatively large concentrations of the contraband to obtain detection. Because the contraband is inside a package, often with intent to conceal from sniffer dogs, detectable concentrations are typically below current MS detection levels.
Mass spectroscopy currently enjoys a premier position in forensics because it is one of the few analytical technologies that can unambiguously identify chemical analytes. A critical issue in forensics, however, is the limited amount of sample available for testing. Higher sensitivity MS technology may significantly improve forensic science and result in higher conviction rates. Forensic applications are also not just limited to law enforcement agencies, but are also of keen interest in the intelligence community for treaty compliance and rogue state monitoring for weapons of mass destruction, parents and management searching rooms, offices, factories, and schools for illicit drugs.
Industrial environmental monitoring is another major application area for mass spectrometers both from environmental protection and industrial hygiene perspectives. Emerging applications include food and beverage safety and quality control as well as odor control in buildings and commercial airlines.
Another application requiring higher sensitivity MS technology is in the collection of biological information (e.g., genomics, proteomics, and metabolomics). Mass spectrometry plays a critical and increasing role in the collection of biological information. The next generation of high throughput and low cost gene sequencing—necessary for the cost effective identification of single nucleotide polymorphisms (SNPs), widespread genotyping for genetic diseases, disease predilection screening, as well as therapeutic tolerance and outcome prediction—is built on MS technology. (Butler, J. M., J. Li, J. A. Monforte, and C. H. Becker, “DNA typing by mass spectrometry with polymorphic DNA repeat markers”; U.S. Pat. No. 6,090,558, (Jul. 18, 2000); Schmidt, G., A. H. Thompson, R. A. W. Johnstone, “Compounds for mass spectrometry comprising nucleic acid bases and aryl ether mass markers”; Eur. Patent 1042345A1 (Oct. 11, 2000); Schmidt, G., A. H. Thompson, R. A. W. Johnstone, “Mass label linked hybridisation probes,” Eur. Patent 979305A1 (Feb. 16, 2000); Koster, H., “DNA sequencing by mass spectrometry,” U.S. Pat. No. 6,194,144 (Feb. 27, 2001)). All protein identification and sequencing is now almost exclusively conducted by MS. Peptide fingerprinting and de novo peptide sequencing by tandem MS are almost universally practiced nonproprietary methods. (Shevchenko, A., et al., “Linking genome and proteome by mass spectrometry: Large-scale identification of yeast proteins from two dimensional gels,” Proc. Natl. Acad. Sci. (USA), 93:14440–14445 (1996); Yates, J. R., S. Speicher, P. R. Griffin, and T. Hunkapiller, “Peptide mass maps: a highly informative approach to protein identification,” Anal. Biochem., 214:397–408 (1993)). Even the classic Edman digestion approach has been adapted to the MS (Aebersold, R. et al., Protein Sci., 1:494–503 (1992)) because of the lower sample requirements and increased speed the MS offers. Inverted mass ladder sequencing, an ultra-fast de novo protein sequencing method, (Schneider, L. V. et al., “Methods for determining protein and peptide terminal sequences” Provisional Patent Nos. 60/242398 and 60/242165 (2000)) also uses an ESI-TOF MS. Stable isotope ratio MS is being used for generating metabolic data (metabolomics). (Schneider, L. V. et al., “Metomics,” U.S. patent application Ser. No. 09/553424 (2000)). The recent invention of mass spectrometer-based differential display techniques, such as isotope coded affinity tags (ICAT™) (Aebersold, R. H., et al., WO 00/11208 (Mar. 2, 2000)) and isotope differentiated binding energy shift tags (IDBEST™) (Schneider, L. V. et al., WO 01/49951 (Aug. 29, 2002); Hall, M. P. et al., poster presented at the Sienna Conference, Siena, Italy (Sep. 1–5, 2002)), allows the direct quantitative comparison of relative protein expression between two or more samples based on the ratio of stable isotopes in the mass spectrometer. All these applications depend on the MS for detection and are crippled by the detection efficiency of the MS. In addition to the generation of primary bioinformatic data, MS is playing a pivotal role in combinatorial chemistry and high throughput drug library screening. (Sugarman, J. H., R. P. Rava, and H. Kedar, “Apparatus and method for parallel coupling reactions,” U.S. Pat. No. 6,056,926 (May 2, 2000); Schmidt, G., A. H. Thompson, and R. A. W. Johnstone, “Mass label linked hybridisation probes,” EP979305A1 (Feb. 16, 2000); Van Ness, J., Tabone, J. C., H. J. Howbert, and J. T. Mulligan, “Methods and compositions for enhancing sensitivity in the analysis of biological-based assays,” U.S. Pat. No. 6,027,890 (Feb. 22, 2000)).
The limiting factor in virtually all these MS bioinformatic applications is the amount of available sample. For example, the protein detection limits in 2-D gel electrophoresis are about 0.2 ng (by silver staining) (Steinberg, Jones, Haugland and Singer, Anal. Biochem., 239:223 (1996)) to about 0.05 fmol (by fluorescent staining) (Haugland, R. P., “Detection of proteins in gels and on blots,” in Handbook of fluroescent probes and research chemicals, Spence, M. T. Z (ed.), 6th ed. (Molecular Probes, Inc., Eugene, Oreg., 1996)), assuming a nominal 40 kDa protein. As little as 1 fmol of unlabeled protein is needed for detection (by UV detection) (Beckman Instruments, “eCAP SDS 200: Fast, reproducible, quantitative protein analysis,” BR2511B (Beckman Instruments, Fullerton, Calif., 1993)) and as little as 1–10 zmol of fluorescently-labeled proteins is needed (by laser-induced fluorescence, LIF) (Beckman Instruments, “P/ACE™ Laser-induced fluorescence detectors, BR-8118A” (Beckman Instruments, Fullerton, Calif., 1995); Harvey, M. D., D. Bandilla, and P. R. Banks, “Subnanomolar detection limit for sodium dodecyl sulfate-capillary gel electrophoresis using a fluorogenic, noncovalent dye,” Electrophoresis, 19:2169–2174 (1998)) can be detected in capillary electrophoretic separations. However a minimum of 0.1 fmol and more typically up to 100 fmol of a protein is required for MS sequencing.
Arguably, high resolution mass spectrometry (MS) has the greatest potential chemical discrimination capacity (50–100,000+ amu mass range with 1 ppm mass accuracy, single ion counting at the ion detector, and the broadest applicability of any analytical chemistry technology. However, mass spectrometers generally exhibit poor detection efficiency for organic samples, often in the range of 0.001–100 parts per million (ppm), or about 0.001–100 fmole (about 106–1011 starting molecules) depending on the ionization method and mass analyzer used.
Mass spectrometry (MS) fundamentally consists of three components: ion sources, mass analyzers, and ion detectors. The three components are interrelated; some ion sources may be better suited to a particular type of mass analyzer or analyte. Certain ion detectors are better suited to specific mass analyzers. Electrospray (ESI) and matrix assisted laser-induced desorportion (MALDI) ionization sources are widely used for organic molecules, particularly biomolecules and are generally preferred for the ionization of non-volatile organic species. ESI is widely practiced because it can be readily coupled with liquid chromatography and capillary electrophoresis for added discrimination capability. MALDI techniques are widely practiced on large molecules (e.g., proteins) that can be difficult to solubilize and volatize in ESI. The principle advantage of MALDI is the small number of charge states that arise from molecules with a multiplicity of ionizable groups. The principle disadvantage of the MALDI is ion detector saturation with matrix ions below about 900 amu. With the advent of micro/nano-ESI sources these two ion sources generally exhibit similar detection sensitivities over a wide range of organic materials.
The detection efficiency (ηd, equation 1) of any MS is determined from the product of the ionization efficiency (ηi, equation 2) and the transmission efficiency (ηt, equation 3). For simplicity the efficiency of the detector element is lumped into the transmission efficiency.
                              η          d                =                                            η              i                        ⁢                          η              t                                =                                          ⁢                                    ion              ⁢                                                          ⁢              current              ⁢                                                          ⁢              at              ⁢                                                          ⁢              the              ⁢                                                          ⁢              detector                                      rate              ⁢                                                          ⁢              of              ⁢                                                          ⁢              molecule              ⁢                                                          ⁢              liberation              ⁢                                                          ⁢              from              ⁢                                                          ⁢              the              ⁢                                                          ⁢              source                                                          (        1        )                                          η          i                =                                  ⁢                              ion            ⁢                                                  ⁢            current            ⁢                                                  ⁢            from            ⁢                                                  ⁢            the            ⁢                                                  ⁢            source                                rate            ⁢                                                  ⁢            of            ⁢                                                  ⁢            molecule            ⁢                                                  ⁢            liberation            ⁢                                                  ⁢            from            ⁢                                                  ⁢            the            ⁢                                                  ⁢            source                                              (        2        )                                          η          t                =                              ion            ⁢                                                  ⁢            current            ⁢                                                  ⁢            at            ⁢                                                  ⁢            the            ⁢                                                  ⁢            detector                                ion            ⁢                                                  ⁢            current            ⁢                                                  ⁢            from            ⁢                                                  ⁢            the            ⁢                                                  ⁢            source                                              (        3        )            
The overall detection efficiency in MS is difficult to measure with good precision. There are a large number of factors that may affect ion formation, collection, transmission, and detection, which are difficult to reproduce exactly from day to day, MS to MS, and lab to lab.
Conventional wisdom for ESI mass spectrometry is that virtually all the losses occur during ion transmission into and through the mass analyzer and that ionization efficiency is close to 100%. This assumption is based on two observations: 1) total ion current measurements from the spray tip and at various positions inside the mass analyzer, and 2) most analytes exhibit multiple charge states.
Smith and coworkers (Tang, K. et al., Anal. Chem., 73:1658–1663 (2001)) measured the actual total ion current (TIC) from ESI microspray tips to be about 150 nA at a 1 μl/min flow rate of a typical biomolecular sample matrix (50:50:1 methanol:water:acetic acid). Using a similar measurement apparatus, but with octanol doped with sulfuric acid as the sample matrix, de la Mora and Loscertales (De la Mora, J. F. and I. G. Loscertales, J. Fluid Mech., 260:155–184 (1994)) reported ion currents of between 50–280 nA (at 1 μl/min flow rates) that varied with the sulfuric acid concentration (between 0.3 and 3%, respectively). Both of these results translate to between 104 to 107 unbalanced charges per drop, assuming 1 to 10 μm drops, respectively (Table 1 below). However, de la Mora and Loscertales observed that the measured ion current was 4 times their theoretical maximum and attributed this difference to electron conductance in the apex region of the jet rather than to ion convection by droplets crossing the gap. If true, then the actual number of charges per drop may be somewhat lower than the total ion current data suggests.
Smith and coworkers (Smith, R. D., et al., Anal. Chem., 62:882–899 (1990)) also attempted to estimate transmission efficiency by measuring the TIC striking a detection plate placed at various positions along the ion path in the mass analyzer. They concluded that transmission efficiency accounted for the vast majority of ion loss culminating in poor detection efficiency. The existence of multiple charge states, or more particularly that the distribution in charge states is not centered about a single charge state, is the second observation supporting complete ionization. If there were a paucity of charge, then few charge states should be seen.
Unlike ESI, it is generally accepted that ionization efficiency in MALDI is poor. One argument for this is the lack of highly-charged species generated from analytes with a large number of readily ionizable sites. For example, in positive ion mode, proteins generally ionize to generate species with +1 or +2 charges only, even though there are generally many more basic residues (i.e., Arg, Lys, and His). Levis (Levis, R. J. , Annu. Rev. Phys. Chem., 45:483–518 (1994)) has clearly demonstrated, by collecting and analyzing all the material liberated from the target by the ionization laser, that MALDI ionization efficiencies are very low and that a large amount of neutralized material is ablated from the MALDI surface by the laser desorption process. This assertion is also supported by the results of Brune and coworkers (Brune, D. C. et al., poster presented at the Amer. Soc. Mass Spectro. Ann. Mtg., Chicago, Ill. (May 27–30, 2001)) who report the optimization of negative ion MALDI matrices based on the gas phase basicity of the matrix molecule. They invoked a gas phase proton transfer argument to explain why higher analyte efficiencies were seen with more basic matrices in MALDI.